
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
PM7341 S/UNI-IMA-84
DATASHEET
PMC-2000223
ISSUE 4
INVERSE MULTIPLEXING OVER ATM
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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10.3 Link FIFOs
In the transmit direction, per link FIFOs exist to provide an elastic store to ensure
proper operation. The number of cells in the FIFO at any particular time varies
as a function of the CDV introduced by the insertion of stuff cells and ICP cells,
the effects of the physical link interface, and the smoothing of data from the ATM
layer for group level CDV. During operation, these FIFOs are loaded a cell at a
time and emptied in a TDM fashion. The Link FIFOs in the transmit direction are
to be 8 cells deep.
In the receive direction, the link FIFOs serve as an interface between the TDM
domain and the ATM cell domain. Cells are gathered in the FIFO for each link
and then burst out to the external memory by the RDAT. The Link FIFOs in the
receive direction are 2 cells deep.
A diagnostic loopback capability is provided to loopback data from the receive
link FIFOs to the transmit link FIFO; this will loopback all links when enabled.
10.4 TC Layer
10.4.1 TX TC Layer (TTTC)
The TX TC layer (TTTC) performs the TC layer functions. These functions consist
of optional HEC generation, optional payload scrambling, and cell-rate
decoupling through physical layer (idle) cell insertion.
This function removes data byte by byte from the per-link FIFOs as required to
provide data to the physical layer function. When the physical layer function
needs a byte of data, it will request data from the (TTTC). The TTTC will then
read a byte of data from the per-link FIFO. If that byte is the first byte of a cell
and the logical channel FIFO is empty, the TTTC will format the next 53 bytes as
a physical layer (idle) cell. If the byte is the fifth byte of a cell, the byte is
optionally overwritten by the CRC-8 calculation over the previous four bytes for
that logical channel. The sixth through 53
rd
bytes may be scrambled by a x
43
+1
self-synchronous scrambler.
Note: Since the Link FIFOs are cell based, an underrun cannot happen in the
middle of a cell.